Before Darwin, scientists believed that all life had been specially created Once created, organisms could not change



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Balance of Nature

  1. Before Darwin, scientists believed that all life had been specially created

  2. Once created, organisms could not change

  3. Argument from Design - all of nature is designed in accord with a predetermined, benevolent, and supernatural plan

  1. The complexity and diversity of life results from the unfolding of the divine plan behind nature

  2. The word evolution comes from the Latin evolutio = to unroll

  3. Evolution simply meant the unfolding of the predetermined divine plan

  1. Nature was designed by the Creator to guide and instruct mankind

  2. Nature was like a giant jigsaw puzzle

  3. Assemble the pieces and you would see the divine plan and our place in nature

  1. Natural Theology - philosophical school started by John Ray (1627-1705)

  2. Ray - Wisdom of God in Creation (1691)

  1. Natural Theology maintained that every living thing was perfectly adapted to its way of life

  2. Fit between form and function in organisms was designed by the Creator

  3. Natural adaptation and design proved the existence of a wise and benevolent deity

  1. Natural Theology was revived in the 19th Century by William Paley (1741-1805)

  2. Paley’s Natural Theology (1802), greatly influenced Charles Darwin

  3. Paley viewed the Creator as the master gardener, or cosmic watchmaker

  1. The natural theologians were fascinated by the balance of nature

  2. What cosmic forces kept the delicate balance between organisms and their environment?

  3. Darwin also worried about this - why aren’t we up to our neck in house flies??

  1. Populations of most organisms have an incredible potential for growth

  2. Consider the lowly house fly

  1. Populations of most organisms have an incredible potential for growth

  2. Consider the lowly house fly

  3. House flies have seven generations per year, 120 flies per generation - what would happen if they all lived?

  1. Some bacteria can divide as often as once every 20 minutes

  2. At that rate, a single bacterium, in just two days, would outnumber the entire human race – and that includes everyone who has ever lived - with a population of 2144 bacteria!




  1. In four days = 2288 , a number so large it is literally cosmic

  2. That’s more than the estimated number of protons in the known universe (a mere 2266)




  1. Forget bacteria, and nasty little flies, what about cute little robins?

  1. A female robin lays four eggs per clutch

  1. A female robin lays four eggs per clutch

  2. She can lay two clutches in one year

  1. A female robin lays four eggs per clutch

  2. She can lay two clutches in one year

  3. What if all 8 baby robins survived?

  1. A female robin lays four eggs per clutch

  2. She can lay two clutches in one year

  3. What if all 8 baby robins survived?

  4. End of one year = 64 robins

  1. End of ten years = 24,414,060 robins

  2. At the end of 30 years, the entire planet Earth would be buried under a blanket of robins 4.5 miles thick !!!

  1. Needless to say, most organisms don’t live long enough to reproduce

  2. There are many limiting factors in nature that regulate the growth of populations

  1. Limiting factors can act from outside the population - extrinsic limiting factors - can be physical (abiotic) factors:

  1. Sunlight

  2. Water

  3. Nutrients

  4. Food

  5. Resources

  1. Limiting factors can act from outside the population - extrinsic limiting factors - can be biotic (living) factors:

  2. Competition

  3. Predation

  4. Symbiosis

  1. The pioneering ecologist Charles Elton proposed that equilibrium in nature resulted mainly from a balance of interactive forces, especially competition and predation

  2. Elton also proposed that food supply was a critical limiting factor for most species

  1. Limiting factors can also act from inside the population - intrinsic limiting factors

  2. Changes in reproductive physiology

  3. Changes in behavior

  1. Most populations are regulated by a combination of limiting factors - they never reach their full reproductive potential

  1. Limiting factors can act in proportion to how dense the population has become - density-dependent limiting factors

  2. Limiting factors can have the same effect regardless of how dense the population has become (forest fires, tidal waves) - density-independent limiting factors

  1. These limiting factors act as a feedback loop to keep populations in equilibrium with their environment

  1. Are these limiting factors extrinsic or intrinsic, density dependent or independent?

  2. Forest fires

  3. Food

  4. Diseases or parasites

  5. Predators

  1. Reproduction in white-tailed deer is related to food supply, an extrinsic limiting factor

  2. Food supply is also a density-dependent limiting factor - it limits populations with respect to the density of the population

  3. Limited food supply affects the ability of the deer to reproduce

  1. What effect does hunting have on the population?

  2. That depends on Bubba….

  1. The Bubba Effect - when a road is put through a forest, an area on either side will rapidly degrade - the distance depends on how far Bubba is willing to walk with a 6 pack of beer under one arm, and a shotgun under the other…


  2. Density-dependent regulation can be subtle

  3. Populations can still regulate themselves with respect to density, even when their resources are unlimited

  1. John B. Calhoun’s classic experiments in the 1950’s with Norway rats have some serious implications for human populations

  2. Calhoun determined the level of population density at which rats started to show signs of stress, then let the populations grow to twice that density

  1. He provided a rat utopia, with unlimited food, water, and nesting materials

  2. Everything except more elbow room…

  1. Rats are great mothers, build comfortable nests, carry their babies away from danger by the scruff of the neck, just like momma cats

  2. Male rats have a stable pecking order, rarely bother the females or young

  1. As densities increased, Calhoun observed a gradual breakdown in maternal care

  2. Females stopped building nests

  3. Females scattered or abandoned their infants, leaving them to be killed and eaten by other rats

  1. As densities increased, Calhoun observed a gradual breakdown in male behavior

  2. Males fought more often, became hyperaggressive

  3. Males became socially catatonic, withdrawn

  4. Males became cannibalistic

  5. Males showed homosexual behavior

  1. When the experiment was complete, Calhoun analyzed the rats’ brains

  2. Found gross changes in the adrenal cortex, a center for production of several key hormones driving aggressive and reproductive behavior

  1. Later trials found evidence of brain changes at relatively low levels of overcrowding

  2. These same brain mechanisms are found in our own brains

  3. Will we end up like Calhoun’s rats??

  1. Ladybugs love to eat aphids

  2. Temperature constant - populations are in balance with one another

  3. Increase temperature - increase ladybug’s metabolism - eats all the aphids, then starves

  4. Decrease temperature - ladybugs slow down, can’t keep up - aphid populations grow rapidly

  1. Sometimes predators are so important that the proper functioning of the ecosystem depends upon them

  2. Remove the predator, the system crumbles, like removing the keystone from an arch

  1. Off the coast of California are vast underwater forests of kelp

  2. Sea urchins graze on the kelp

  3. Sea otters feed on the sea urchins

  1. Sea otters were decimated by hunting

  2. Sea urchin populations exploded, ate up all the kelp - otters were keystone predators

  3. Kelp ecosystem collapsed, coastal erosion

  4. Otters were protected, back to normal

  1. Jane Lubchenko studied the marine snail Littorina littorea, which likes to eat a green algae called Enteromorpha

  2. Enteromorpha is a superior competitor, which dominates the tide pools where it occurs

  1. What effect does Enteromorpha have on species diversity in tide pools?

  2. At high densities of Enteromorpha, species diversity is low because the better competitor crowds out the competition

  1. When there are many snails grazing on Enteromorpha, species diversity is high, because other species have room to grow

  2. If there are too many snails, species diversity is low, the snails eat everything!

  3. Species diversity is highest at intermediate densities of grazing snails

  1. Without predators, many populations would explode out of control

  2. Prickly pear cactus (Opuntia) was introduced to Australia by a careless gardener

  3. In the absence of any natural enemies, the cactus began to devour Australia

  1. At its peak, cactus covered 43,320 square miles of Australia!!

  2. Was gobbling up the continent at the rate of 1,544 square miles per year

  1. Opuntia is held in check in its native South American habitat by a moth that eats it

  2. Introduced cactus-eating moth, cactus quickly brought under control


  3. Moth introduced in the Caribbean in 1956, soon acquired a taste for other native cacti

  4. Moth invaded Louisiana in 2009

  5. Could become a serious pest in Texas and esp. Mexico, where 50,000 farmers raise prickly pear cactus for food (it’s even on the flag!)



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